Ground Geometries Research Articles

Investigation of the Ligand-Field States of the Hexaammine Cobalt(III) Ion with Quantum Chemical Methods

The ligand-field (LF) transition energies of the Co(NH3)6(3+) ion have been computed with multiconfiguration quasidegenerate second-order perturbation theory (MCQDPT2). The water solvent was treated with the polarizable continuum model (PCM), and the environment in crystals was modeled by the Co(NH3)6·Cl4(-) complex. The Co-N bond lengths, calculated for the hydrated cation and the Co(NH3)6·Cl4(-) model compound, agree with those in the crystal structures. The vertical transition energies agree with experiment, whereby those based on Co(NH3)6·Cl4(-) are more accurate than those for the hydrated ion. The 0-0 transitions were based on the OPBE geometries of ground and excited (1)T1g, (3)T1g, (5)T2g states of the hydrated ion. The (3)T1g state is the lowest excited state; the (5)T2g state lies higher by >0.6 eV.

Modulation of electronic structures of thienylene vinylene oligomers by substituents and solvents: ground and excited states

AbstractThe effects of substituents on the electronic structures of di(thienylene vinylene) (2TV) in ground and excited states are studied using density functional theory (DFT) and time‐dependent DFT (TD‐DFT), respectively. A representative set of electron donating groups (amino, methoxy and methyl) and withdrawing groups (acetylene, cyano and nitro) are introduced on the vinylene and thienyl moieties to investigate the influence of substituents. Bulk solvent effects are also taken into account by means of the polarizable continuum model (PCM). In contrast to the aromatic structures of 2TV and its derivatives in their ground (S0) states, the electronic structures of first singlet excited (S1) states are rather delocalized. The electron‐donating/withdrawing capability, position and number of substituents are important factors in tuning the vertical S0 → S1 absorption energies and S1 → S0 emission energies of 2TV derivatives. The NO2‐ and NH2‐substituents exert significant effects on the geometries of both ground and excited states and hence the absorption and photoluminescence (PL) emission spectra. The solvent polarity introduces modest influence on the excitation energies for most of the 2TV derivatives. But the absorption and PL emission spectra of nitro‐substituted 2TV exhibit noticeable red shifts as the medium polarity increases. Copyright © 2008 John Wiley & Sons, Ltd.

Theoretical Studies of Blue-Emitting Iridium Complexes with Different Ancillary Ligands

The structural and electronic properties of two heteroleptic iridium complexes Ir(dfppy)2(pic) (FIrpic) and Ir(dfppy)2(acac) (FIracac) have been investigated theoretically, where dfppy = 2-(2,4-difluorophenyl) pyridine, pic = picolinic acid, and acac = acetoylacetonate. The geometries of ground and excited states are optimized at PBE0/LANL2DZ and CIS/LANL2DZ levels, respectively. Time-dependent density functional theory (TDDFT) method is employed to explore the absorption and emission properties. In the ground state, the highest-occupied molecular orbital has a significant mixture of metal Ir(d) and dfppy(pi), the lowest-unoccupied orbital locates primarily on pi* of pic for FIrpic and pi* of dfppy for FIracac. The luminescence of each complex originates from the lowest triplet excited state, which is assigned to the mixing of metal-to-ligand charge transfer and intraligand charge transfer characters. The effects of ancillary ligands pic and acac on absorption and emission spectra are observed by analysis of TDDFT results. The connection between the nature of excited states and the behavior of the complexes with different ancillary ligands is elucidated.

Experimental and DFT Studies: Novel Structural Modifications Greatly Enhance the Solvent Sensitivity of Live Cell Imaging Dyes

Structural modifications of previously reported merocyanine dyes (Toutchkine, A.; Kraynov, V.; Hahn, K. J. Am. Chem. Soc. 2003, 125, 4132-4145) were found to greatly enhance the solvent dependence of their absorbance and fluorescence emission maxima. Density functional theory (DFT) calculations have been performed to understand the differences in optical properties between the new and previously synthesized dyes. Absorption and emission energies were calculated for several new dyes using DFT vertical self-consistent reaction field (VSCRF) methods. Geometries of ground and excited states were optimized with a conductor-like screening model (COSMO) and self-consistent field (SCF) methods. The new dyes have enhanced zwitterionic character in the ground state and much lower polarity in the excited state, as shown by the DFT-VSCRF calculations. Consistently, the position of the absorption bands are strongly blue-shifted in more polar solvent (methanol compared to benzene), as predicted by the DFT spectral calculations. Inclusion of explicit H-bonding solvent molecules within the quantum model further enhances the predicted shifts and is consistent with the observed spectral broadening. Smaller but significant spectral shifts in polar versus nonpolar solvent are predicted and observed for emission bands. The new dyes show large fluorescence quantum yields in polar hydrogen-bonding solvents; qualitatively, the longest bonds along the conjugated chain at the excited S1 state minimum are shorter in the more polar solvent, inhibiting photoisomerization. The loss of photostability of the dyes is a consequence of the reaction with and electron transfer to singlet oxygen, starting oxidative dye cleavage. The calculated vertical ionization potentials of three dyes I-SO, AI-SO(4), and AI-BA(4) in benzene and methanol are consistent with their relative photobleaching rates; the charge distributions along the conjugated chains for the three dyes are similarly predictive of higher reaction rates for AI-SO(4) and AI-BA(4) than for I-SO. Time-dependent DFT calculations were also performed on AI-BA(4); these were less accurate than the VSCRF method in predicting the absorption energy shift from benzene to methanol.

A quantum mechanical polarizable continuum model for the calculation of resonance Raman spectra in condensed phase

In this paper we present two computational strategies to simulate resonance Raman spectra of solvated molecules within the framework of the polarizable continuum model (PCM). These two strategies refer to two different theoretical approaches to the RR spectra, namely the transform theory and the short-time dynamics. The first is based on the explicit detemination of the mimimum geometries of ground and electronically excited states, whereas the second only needs to know the Franck–Condon region of the excited state potential energy surface. In both strategies we have applied the recent advances achieved in the QM description of excited state properties and geometries of solvated molecules. In particular, linear response approaches such as CIS and TDDFT, and their extensions to analytical gradients, are here used to evaluate the quantities required to simulate resonance Raman spectra. The methods have been applied to the study of solvent effects on RRS of julolidine malononitrile (JM). The good agreement found between the calculated and experimental RR spectra seems to confirm the reliability of the computational strategies based on the PCM description.

Terrestrial-LIDAR Visualization of Surface and Structural Deformations of the 2004 Niigata Ken Chuetsu, Japan, Earthquake

Following the 23 October 2004 Niigata Ken Chuetsu, Japan, Mw 6.6 earthquake, LIDAR (light detection and ranging) technology was used to create ultra high-resolution three-dimensional digital terrain models of the earthquake damage. Two reconnaissance teams traveled with tripod-mounted LIDAR that allowed for the rapid collection of post-earthquake failure geometries of ground, structures, and lifelines prior to modification by post-disaster recovery efforts and natural processes, with range accuracies of approximately 2.5 cm and targets illuminated up to 400–700 m from the sensor. LIDAR offers several benefits: (1) detailed failure morphologies of damaged ground and structures, measured remotely and in a way not feasible by conventional means; (2) exploration and visualization of damage on a computer screen is enabled, in orientations and scales that were previously impossible, providing better definition of the failure surfaces, deformation patterns, and morphologies required for understanding failure modes; and (3) archived ultra-high-resolution data for evaluation of analytical and numerical models of deformation. High-resolution images and movies of LIDAR data can be viewed at http://walrus.wr.usgs.gov/geotech/Niigata/ and the online pages of Earthquake Spectra.

Theoretical investigation of the chemical bonds in metal chelates as emitting material for OLED

Geometries of ground (S 0) states of mer-tris(8-hydroxyquinolinato)metal (Mq3, M = Al 3+, Ga 3+, In 3+, Tl 3+) are optimized by B3LYP/6-31G(d) methods. In order to investigate the distribution for unequal individual ligands in mer-Mq3 as key materials for OLED, the bonding interactions either between the metal fragment Mq2 and a single ligand q have been analyzed with the energy decomposition scheme. The bonding analysis has been carried out at the BP86 level using TZ2P quality basis functions with small core. All the calculated results suggest that the HOMOs for Mq3 are mainly localized on A-quinolate ligand, which are responsible to the optical characteristics. The calculated phenomena about optical and structural properties of mer-Mq3 can be simply traced back to the lowest electrostatic attractive interaction energy between fragments A-quinolate ligand and the Mq2 due to the special arrangement, resulting in the easy of geometry relaxation for excited state or interaction with metal atoms.

Quantum chemical analysis of the chemical bonds in Mq3 (M = Al III, Ga III) as emitting material for OLED

Geometries of ground and first excited states of mer-tris(8-hydroxyquinolinato)metal (Mq3, M = Al III, Ga III) are optimized at B3LYP/6-31G(d) and CIS/6-31G level, respectively. In order to investigate the difference for individual ligands in mer-Mq3, the energy partitioning analysis has been carried out at the BP86 level using TZ2P basis functions for the bonding interactions between each fragment Mq2 and single ligand q. HOMO and LUMO distribution fashion can be traced back to the lowest electrostatic attractive and highest orbital interaction energy between fragments A-quinolate ligand and Mq2 and B-ligand and Mq2, respectively.

Tautomerization in the ground and first excited singlet states of phenyl-lapimidazole

Ground and first excited singlet state tautomerization of phenyl-lapimidazole (PLI) was studied by absorption, fluorescence, time-resolved fluorescence, NMR spectroscopy and molecular orbital calculations. In the non-protic solvents cyclohexane and DMSO, PLI is observed by NMR, static and time-resolved fluorescence spectroscopy to exist in two different tautomeric forms, while in protic solvents a rapid equilibrium takes place above 50°C and only one spectroscopic species is observed. The overlapped fluorescence excitation and emission spectra were each decomposed into pure components by the principal component analysis-self modeling chemometric method. Ab initio (HF, CIS and CAS) and hybrid DFT (B3LYP) quantum chemical methods have been used to calculate the ground and first excited singlet state geometries, the vertical transition energies and the NMR chemical shifts were used to assign and rationalize the spectral data.

Theoretical study of the electronic states of niobium trimer (Nb3) and its anion (Nb3−)

Geometries and energy separations of the various low-lying electronic states of niobium trimer (Nb3) and its anion (Nb3−) with triangular and linear structural arrangements have been investigated. The complete active space multiconfiguration self-consistent field method followed by multireference singles plus doubles configuration interaction (MRSDCI) that included up to 48 million configuration spin functions have been used to compute several electronic states of these clusters. The geometries of ground and excited states of Nb3 and Nb3− are triangular. The ground states of both Nb3 (2B1) and Nb3− (1A1) have been found to be of low spin. The low-lying electronic states with degenerate symmetries in the D3h group are distorted to the C2v structure (from the ideal D3h) due to the Jahn–Teller effect. On the basis of the energy separations of our computed electronic states of Nb3, we have assigned the observed photoelectron spectrum of Nb3−. We have also compared our MRSDCI results with density functional calculations. The electron affinity, ionization potential, dissociation and atomization energies of Nb3 have been calculated and the results have been found to be in excellent agreement with the experiment.

Electronic Structure and Barrier to Internal Rotation of Sulfenamide, (N,N-Disubstituted and N-aryl)benzsulfenamide and N-aryl Methylsulfenamide Derivatives

The ground geometries of sulfenamide, N,N-disubstituted benzsulfenamide, N-aryl benzsulfenamide, and N-aryl methylsulfenamide derivatives have been examined theoretically. Equilibrium geometries were determined using 6-311G** and 6-311G**+MP2 basis sets for sulfenamide (I) and 6-31G* for the derivatives at the level of RHF method. The calculated geometric parameters of sulfenamide (I)are analyzed. Detalied investigation of the electronic structure of sulfenamide (I) is facilitated via the analysis of charge density maps (total, HOMO, and LUMO). The substituents effect on geometry, energetics, and relative stability of the studied derivatives also are analyzed. The investigated nature of the hindered rotation about the N—S bond of some N,S-disubstituted sulfenamide derivatives has been computed theoretically using 6-31G* and 6-31G*+MP2 theory level.

Low-lying electronic states of difluorodioxirane

The electronic structures of the A11 ground state and several low-lying excited states of difluorodioxirane (CF2O2) were investigated using the recently revised, multireference, second-order Generalized Van Vleck Perturbation Theory (GVVPT2) method for the description of electron correlation. Multiconfigurational self-consistent field calculations that included 19 molecular orbitals demonstrated that the electronic structures of all considered states are substantially multiconfigurational, with maximum configuration amplitudes not larger than 0.9. Equilibrium geometries of ground and first excited A11 states and the energetically lowest lying B1 and B2 triplet and singlet states were optimized at the GVVPT2 level. Additionally, the nature of and vertical excitation energy to the 1 1A2 state are evaluated. Most of the considered states are predicted to be thermodynamically stable relative to their adiabatic dissociation limits and, at least some of them, should be observable in absorption spectra. The O–O bond lengths in the excited states are predicted to be significantly larger (1.94–2.34 Å) than in the ground state (1.578 Å), which contains the longest known bond of this type.

Ground and excited state CASPT2 geometry optimizations of small organic molecules.

A method for computing second-order multiconfigurational perturbation theory (CASPT2) energy gradients numerically has been implemented and applied to a range of elementary organic chromophores, including 1,3 butadiene, acrolein, and two protonated Schiff bases. Geometries of ground and excited states-as well as conical intersections-are compared with the corresponding CASSCF structures, illustrating the effect of including the correction for dynamical electron correlation. It is shown that the differences between the two methods are not readily categorized, but that, while individual changes in bond lengths can be quite large ( approximately 0.01-0.02 A), the natures and CASPT2 energetics of the structures remain similar. Exceptions to this tend to be systems that have a strong ionic character and that are not well described at the CASSCF level. Basis set effects (double- vs. triple-zeta) were examined for a limited number of examples, and found to be quite dramatic at both levels of theory.

Shpol'skii spectroscopy and vibrational analysis of [N]phenylenes

Vibrationally resolved fluorescence spectra of four angular [N]phenylenes were recorded with laser excited Shpol’skii spectroscopy (LESS) in an n-octane matrix at 10 K. In general, the same vibrational frequencies were observed in the fluorescence excitation and emission spectra, indicating that the geometries of ground and electronically excited state are very similar. Because of intensity borrowing from the S2 state, vibrations of two different symmetries were observed in the fluorescence excitation spectra of angular [3]phenylene and zig-zag [5]phenylene. This finding allowed the location of the S2 state for these compounds. DFT calculations (RB3LYP/6-31G*) of the ground state vibrational frequencies were made. The calculated vibrational modes were in reasonably good agreement with the experimental data. A new very low-frequency vibration of approximately 100 cm−1 was predicted and experimentally confirmed for all [N]phenylenes investigated. This vibration seems to be unique for [N]phenylenes and is attributed to an in-plane movement of the carbon backbone.

Laser ionization spectroscopy of Ag(NH3) n clusters

The ionization and the fragmentation properties of Ag(NH3) n heteroclusters have been studied. The measured ion yield spectra near the threshold for complexes withn=2−20 indicate strong differences of the cluster geometry in the electronic ground and in the ionized state, respectively. For smaller clusters (n≤6) in the neutral ground state the nonsolvated Ag atom is localized near the surface of the complex. A modification of the surface structure at the transition to larger clusters (n≥7) is suggested by significant changes in the ionization and fragmentation behaviour. Preliminary results of ab initio calculations confirm the conclusion that due to the correlation interaction between the valence electron and the 4d electrons of the Ag atom no solvation effects occur.

An ab initio Molecular Orbital Study on Effect of Electric Field Strength on Ionic Selectivity of the Sodium Channel

The effect of electric field strength of the selectivity filter on ionic selectivity of the sodium channel has been studied by ab initio molecular orbital calculations. On the basis of the previous method, two kinds of model systems, the FCO2M system and the FCO2M-H2O system, were studied. In this study, a FCO-2 molecule, which is substituted for a HCO-2 molecule, was used as a model for the selectivity filter. This substitution corresponds to weakening the electric field strength of the selectivity filter. For this system, geometries of ground and transition states were optimized with MINI-3 basis sets. The FCO2-H2O system, for which we evaluated thermochemical parameters and cross-sections, is a good model for ionic permeation of the sodium channel. When the results of the FCO2M-H2O system were compared with those of the HCO2M-H2O system, we concluded that permeabilities of Li+ and Na+ are smaller and that of K+ is larger as the field strength of the selectivity filter is weakened. Namely, the permeation of K+ is relatively favorable when the field strength of the selectivity filter is weakened. This conclusion is in accordance with the prediction of Eisenman's theory.

Theoretical prediction of the vibrational spectrum of naphthalene in the first excited singlet state

Complete harmonic force fields have been calculated for the ground state (S0) and the first excited singlet state (S1) of naphthalene using the multiconfiguration self-consistent field (MCSCF) approach. Identical calculations were performed for benzene to test the methodology with already available theoretical and empirical force fields. Two different basis sets were applied (STO-3G and near double-zeta) and all π-orbitals included in the active space. The geometries of ground and excited states were separately optimized. Following the ideas of Pulay, the force constants were scaled before calculating frequencies and normal modes. For the ground states the influence of correlation is discussed by comparison with Pulay’s results. Except for special vibrations where correlation effects turn out to be important, the use of Pulay’s scaling factors leads to a satisfactory description of the in-plane-vibrations. In the case of benzene the calculated frequency shifts between S0 and S1 are in complete qualitative agreement with experimental observations. In the case of naphthalene the new theoretical results suggest several revisions of earlier empirical assignments.

Half-collision studies: Action spectroscopy of electronic energy transfer within the Cd⋅CH4 van der Waals complex

The technique of action spectroscopy has been used to characterize the ground state and excited states of the Cd–CH4 van der Waals complex and to study ‘‘half-collision’’ electronic energy transfer processes within the complex. By tuning one laser pulse in the spectral region near an atomic transition of the free Cd atom and monitoring a predissociation product with a second, delayed laser pulse, it has been possible to obtain ‘‘action’’ spectra which provide information about the excited Cd–CH4 potential surfaces from which electronic energy transfer processes occur. Excitation of the Cd⋅CH4 complex to the red of the Cd(5s5p 3P1 ← 5s5s 1S0) transition while monitoring Cd(5s5p 3P0) produces an anharmonic series of blue-shaded bands which have been assigned to the Cd⋅CH4(A 30+ ← X 10+) transition. The spectra are similar to the A 30+ ← X 10+ laser-induced fluorescence (LIF) spectra of the Cd⋅Ar and Cd⋅Kr complexes, except that there are weaker subbands ∼9 cm−1 to the blue of each main band. Computer simulations of the rotational structure of several of the bands were successful, and spectroscopic parameters for both the X 10+ ground state and the A 30+ upper state are reported. It is postulated that Cd⋅CH4 is a hindered rotor with C3v (facial) geometry in both ground and upper states. Detailed considerations of the possible nuclear-spin isomers of symmetries A, F, and E, using the CH4 hindered-rotor calculations of Endo and Ohshima, lead to an assignment of the main bands as A 30+(E,K′ = 1) ← X 10+(E,K″ = 1) transitions and the subbands as A 30+(2F,K′ = 1) ← X 10+(2F,K″ = 1) transitions. It is suggested that the expected A 30+(F,K′ = 0) ← X 10+(F,K″ = 0) and A 30+(A,K′ = 0) ← X 10+(A,K″ = 0) transitions were not observed because the K′=0 states lack the rotational motion about the C3v axis which can couple the A 30+ (A1) state with the a 30− (A2) state [which correlates with Cd(5s5p 3P0)]. Action spectra of Cd⋅CH4 were also obtained to the red of the Cd(5s5p 1P1 ← 5s5s 1S0) transition when the probe laser was monitoring Cd(5s5p 3P2) products. The severely broadened spectra are consistent with extremely rapid predissociation of the C 1Π1 (C 11) van der Waals state by the repulsive c 3Σ1 (c 31) state, which correlates with Cd(5s5p 3P2) + CH4.

Theoretical studies of the kinetics, thermochemistry, and mechanism of hydrogen-abstraction from methanol and ethanol

The process of hydrogen abstraction from methanol and ethanol has been calculated with ab initio quantum chemical methods with extended basis sets (6-311G) and with the inclusion of correlation up to MP4SDQ. These studies serve as a model of such processes in large molecules of biological importance including the sugar moiety of DNA. A comparison of geometries of ground and transition states optimized at UHF and MP2 levels with the 6-31G basis set shows that the UHF optimized geometries have lower energies at the highest level of theory used (MP4SDQ/6-311G). The transition states occur at a somewhat later stage along the reaction coordinate at the UHF level than at the MP2 level. Energy barriers, along with zero-point energies, were used to calculate the rate constants for H-abstraction from C{sub 1} of methanol and ethanol. Tunneling corrections were applied according to an Eckart treatment of an unsymmetrical unidimensional barrier. The corrected rate constants are in very good agreement with experiment over a wide range of temperatures. The same approach was used to predict the rate constant for the abstraction of the hydrogen from C{sub {beta}} of ethanol, which is not known from experimental measurements. The calculated C-H bond strengths and heatsmore » of reactions are also in good agreement when the correlation energy is scaled according to the MPnSAC approach. The geometric and energetic parameters of the transition states behave according to Hammond`s postulate, i.e., the more exothermic the H-abstraction, the closer is the transition state to the reactants. This relationship suggests that the C-H bond strength is one of the major factors that determine the barrier to H-abstraction. An analysis of the MCSCF wave function constructed from a CAS of three electrons distributed in three orbitals ({sigma}{sub CH}, {sigma}{sub CH}, and the orbital containing the unpaired electron) supports this conclusion. 38 refs., 4 figs., 6 tabs.« less

Photoinduced intramolecular proton transfer in methyl salicylates: a theoretical study with spectroscopic support

Intramolecular proton transfer processes in the ground and excited state of methyl calicylate, methyl 4-hydroxysalicylate and methyl 5-hydroxysalicylate have been studied by CNDO-CI and INDO semiempirical methods in order to determine the existence of a single or a double energy minimum in the corresponding potential curves, as well as to interpret thier photophysical behaviour. Hydrogen-bonding strenghts of ground state and local energy-minized geometries of ground and excited states are also discussed. These studies, combinded with IR and 1H HMR spectral data from methyl 4-methoxysalicylate and methyl 5-methoxysalicylate, support the existence of two different intramolecular hydrogen-bonded forms in the ground state and, at the same time, show that the photphysical behaviour of all these salicylic acid derivatives closely depends on the distance between the two oxygen atoms supporting the bond.